Problem:
Score : 1000 points

Problem Statement

Note the unusual memory limit.

For a rectangular grid where each square is painted white or black, we define its complexity as follows:

If all the squares are black or all the squares are white, the complexity is 0.

Otherwise, divide the grid into two subgrids by a line parallel to one of the sides of the grid, and let c_1 and c_2 be the complexities of the subgrids. There can be multiple ways to perform the division, and let m be the minimum value of \max(c_1, c_2) in those divisions. The complexity of the grid is m+1.

You are given a grid with H horizontal rows and W vertical columns where each square is painted white or black.
HW characters from A_{11} to A_{HW} represent the colors of the squares.
A_{ij} is # if the square at the i-th row from the top and the j-th column from the left is black, and A_{ij} is . if that square is white.

Find the complexity of the given grid.

Constraints

1 \leq H,W \leq 185

A_{ij} is # or ..

Input

Input is given from Standard Input in the following format:

H W
A_{11}A_{12}...A_{1W}
:
A_{H1}A_{H2}...A_{HW}

Output

Print the complexity of the given grid.

Sample Input 1

3 3
...
.##
.##

Sample Output 1

2

Let us divide the grid by the boundary line between the first and second columns.
The subgrid consisting of the first column has the complexity of 0, and the subgrid consisting of the second and third columns has the complexity of 1, so the whole grid has the complexity of at most 2.

Sample Input 2

6 7
.####.#
#....#.
#....#.
#....#.
.####.#
#....##

Sample Output 2

4

Solution in Rust (1.15.1) (judge verdict: Accepted)

use std::cmp::max;
use std::cmp::min;

fn read<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim()
        .split_whitespace()
        .map(|e| e.parse().ok().unwrap())
        .collect()
}

fn main() {
    let hw = read_vec::<usize>();
    let h = hw[0];
    let w = hw[1];
    let mut masu: Vec<Vec<char>> = Vec::with_capacity(h);
    for _i in 0..h {
        let row = read::<String>().chars().collect::<Vec<char>>();
        masu.push(row);
    }
    let c = calc(h, w, &masu);
    println!("{}", c);
}

/**
 * dp1[r1][r2][c1][f]: [r1, r2), [c1, c2)の複雑度がf以下となる最大のc2
 * dp2[c1][c2][r1][f]: [r1, r2), [c1, c2)の複雑度がf以下となる最大のr2
 * メモリ節約のため、fについては現在と一個前の2個分だけ領域を確保する。
 */
fn calc(h: usize, w: usize, masu: &Vec<Vec<char>>) -> i32 {
    let max_f = get_max_f(h, w) + 1; // exclusive
    let mut dp1 = vec![vec![vec![vec![0; w]; h+1]; h]; 2];
    let mut dp2 = vec![vec![vec![vec![0; h]; w+1]; w]; 2];

    //println!("max_f = {}", max_f);

    //println!("Init");
    // Init dp1 A (f=any, r2=r1)
    for f in 0..2 {
        for r1 in 0..h {
            for c1 in 0..w {
                dp1[f][r1][r1][c1] = w as u8;
            }
        }
    }

    // Init dp2 A (f=any, c2 = c1)
    //println!("Init dp2 A");
    for f in 0..2 {
        for c1 in 0..w {
            for r1 in 0..h {
                dp2[f][c1][c1][r1] = h as u8;
            }
        }
    }

    // Init dp1 B (f=0, r2 = r2 + 1)
    for r1 in 0..h {
        let r2 = r1 + 1;
        let mut c1 = 0;
        let mut c2 = c1;
        while c1 < w {
            let v = masu[r1][c1];
            // move c2
            while c2 < w && masu[r1][c2] == v {
                c2 += 1;
            }
            // move c1
            while c1 < c2 {
                dp1[0][r1][r2][c1] = c2 as u8;
                //println!("dp1[0][{}][{}][{}] = {}", r1, r2, c1, c2);
                c1 += 1;
            }
        }
    }

    // Init dp2 B (f=0, c2 = c1 + 1)
    //println!("Init dp2 B");
    for c1 in 0..w {
        let c2 = c1 + 1;
        let mut r1 = 0;
        let mut r2 = r1;
        while r1 < h {
            let v = masu[r1][c1];
            // move r2
            while r2 < h && masu[r2][c1] == v {
                r2 += 1;
            }
            // move r1
            while r1 < r2 {
                dp2[0][c1][c2][r1] = r2 as u8;
                r1 += 1;
            }
        }
    }

    // Init dp1 C (f=0, r2 > r2 + 1)
    //println!("Init dp1 C");
    for r1 in 0..h {
        for r2 in r1 + 2..h + 1 {
            let mut c1 = 0;
            while c1 < w {
                // get c2
                let r2_c1 = dp2[0][c1][c1 + 1][r1] as usize;
                if r2_c1 < r2 {
                    // no one-color region
                    dp1[0][r1][r2][c1] = c1 as u8;
                    c1 += 1;
                } else {
                    // get c2
                    let c2_prev = dp1[0][r1][r2 - 1][c1] as usize;
                    let c2_r2 = dp1[0][r2 - 1][r2][c1] as usize; // Calculated in 'Init dp1 B'
                    let c2 = min(c2_prev, c2_r2);
                    // move c1
                    while c1 < c2 {
                        dp1[0][r1][r2][c1] = c2 as u8;
                        c1 += 1;
                    }
                }
            }
        }
    }

    if dp1[0][0][h][0] as usize == w {
        //println!("Bingo!");
        return 0;
    }

    // Init dp2 C (f=0, c2 > c1)
    //println!("Init dp2 C");
    for c1 in 0..w {
        for c2 in c1 + 2..w + 1 {
            let mut r1 = 0;
            while r1 < h {
                let c2_r1 = dp1[0][r1][r1 + 1][c1] as usize;
                if c2_r1 < c2 {
                    dp2[0][c1][c2][r1] = r1 as u8;
                    r1 += 1;                   
                } else {
                    let r2_prev = dp2[0][c1][c2 - 1][r1] as usize;
                    let r2_c2 = dp2[0][c2 - 1][c2][r1] as usize;
                    let r2 = min(r2_prev, r2_c2);
                    while r1 < r2 {
                        dp2[0][c1][c2][r1] = r2 as u8;
                        r1 += 1;
                    }
                }
            }
        }
    }

    if dp2[0][0][w][0] as usize == h {
        return 0;
    }

    // DP
    //println!("DP");
    for f in 1..max_f {
        let f_cur = f %2;
        let f_prev = (f-1) % 2;
        for r1 in 0..h {
            for r2 in r1 + 1..h + 1 {
                let mut c1 = 0;
                let mut c2_yoko = 0;
                while c1 < w {
                    // Tate
                    let c2_tate = tate_wari(w, &dp1, r1, r2, c1, f_prev);

                    // Yoko
                    while c2_yoko < w && yoko_wari(h, &dp2, c1, c2_yoko + 1, r1, f_prev) >= r2 {
                        c2_yoko += 1;
                    }

                    // Choose max
                    let c2 = max(c2_tate, c2_yoko);
                    dp1[f_cur][r1][r2][c1] = c2 as u8;
                    c1 += 1;

                    if c2 == w {
                        // if c2 is w, for all cx > c1, dp1[f][r1][r2][cx] = w.
                        while c1 < w {
                            dp1[f_cur][r1][r2][c1] = w as u8;
                            c1 += 1;
                        }
                    }
                }
            }
        }

        if dp1[f_cur][0][h][0] as usize == w {
            //println!("Bingo!");
            return f as i32;
        }

        for c1 in 0..w {
            for c2 in c1 + 1..w + 1 {
                let mut r1 = 0;
                let mut r2_tate = 0;
                while r1 < h {
                    // Yoko
                    let r2_yoko = yoko_wari(h, &dp2, c1, c2, r1, f_prev);

                    // Tate
                    while r2_tate < h && tate_wari(w, &dp1, r1, r2_tate + 1, c1, f_prev) >= c2 {
                        r2_tate += 1;
                    }

                    // Choose max
                    let r2 = max(r2_yoko, r2_tate);
                    dp2[f_cur][c1][c2][r1] = r2 as u8;
                    r1 += 1;

                    if r2 == h {
                        while r1 < h {
                            dp2[f_cur][c1][c2][r1] = h as u8;
                            r1 += 1;
                        }
                    }
                }
            }
        }

        if dp2[f_cur][0][w][0] as usize == h {
            //println!("Bingo!");
            return f as i32;
        }
    }

    //println!("Fallback");
    return 0;
}

fn tate_wari(
    w: usize,
    dp1: &[Vec<Vec<Vec<u8>>>],
    r1: usize,
    r2: usize,
    c1: usize,
    f_prev: usize,
) -> usize {
    let c_x = dp1[f_prev][r1][r2][c1] as usize;
    if c_x == c1 {
        c1
    } else if c_x >= w {
        w
    } else {
        dp1[f_prev][r1][r2][c_x] as usize
    }
}

fn yoko_wari(
    h: usize,
    dp2: &[Vec<Vec<Vec<u8>>>],
    c1: usize,
    c2: usize,
    r1: usize,
    f_prev: usize,
) -> usize {
    let r_x = dp2[f_prev][c1][c2][r1] as usize;
    if r_x == r1 {
        r1
    } else if r_x >= h {
        h
    } else {
        dp2[f_prev][c1][c2][r_x] as usize
    }
}

fn get_max_f(h: usize, w: usize) -> usize {
    let x = (h as f64).log2().ceil() + (w as f64).log2().ceil();
    return x as usize;
}