Brick Wall

https://leetcode.com/problems/brick-wall/description/

There is a brick wall in front of you. The wall is rectangular and has several rows of bricks. The bricks have the same height but different width. You want to draw a vertical line from thetopto thebottomand cross theleastbricks.

The brick wall is represented by a list of rows. Each row is a list of integers representing the width of each brick in this row from left to right.

If your line go through the edge of a brick, then the brick is not considered as crossed. You need to find out how to draw the line to cross the least bricks and return the number of crossed bricks.

You cannot draw a line just along one of the two vertical edges of the wall, in which case the line will obviously cross no bricks.

Thoughts

观察到遇到的缝最多的即我们要找的那条路。每个缝即一个位置。把缝隙位置存下来, 记下freq。freq最大的即所要找的路。

把墙数减去最大缝的缝数即所求。

Code

class Solution {
    public int leastBricks(List<List<Integer>> wall) {
        Map<Integer, Integer> freq = new HashMap<>();
        int wallLen = 0;
        for (Integer b : wall.get(0)) {
            wallLen += b;
        }
        for (List<Integer> w : wall) {
            int pos = 0;
            for (Integer b : w) {
                pos += b;
                freq.put(pos, freq.getOrDefault(pos, 0) + 1);
            }
        }

        int max = 0;
        for (Integer pos : freq.keySet()) {
            if (pos != wallLen) {
                max = Math.max(max, freq.get(pos));
            }
        }
        return wall.size() - max;
    }
}

Analysis

Errors:

1. 忘了把等于wallLen那条路抛去。

时间复杂度O(n). 空间O(n).