leetcode 221. Maximal Square

class Solution {// 最大正方形面积 public:    int maximalSquare(vector<vector<char>>& matrix) {        if (matrix.empty()) { return 0; }        long len = 0;        vector<vector<int>> dp(matrix.size(), vector<int>(matrix[0].size(), 0));                for (int row = 0; row < matrix.size(); ++row)            for (int col = 0; col < matrix[row].size(); ++col){                if (!col || !row)                    dp[row][col] = matrix[row][col] - '0';                else if (matrix[row][col] != '0'){                    if (dp[row][col - 1] != dp[row - 1][col])                        dp[row][col] = min (dp[row][col - 1], dp[row - 1][col]) + 1;                    else{                        int len = dp[row][col - 1];                        dp[row][col] = len;                     // error: if(dp[row - len][col - len]): tricky! for instance {{0,0},{0,1}}                     // dp[1][1] equals to 1, but we get 0                        if (matrix[row - len][col - len] != '0')                             dp[row][col]++;                    }                }                len = max (len,  static_cast<long>(dp[row][col]));            }        return len * len;     }};

void buildHistogram(vector<string> &matrix, vector<vector<int>>& histogram) { // 构造图for (int col = 0; col < matrix[0].size(); ++col)for (int row = (int)matrix.size() - 1; row >= 0; --row) {if (matrix[row][col] != '0') {histogram[row][col] = 1;if (row < (int)matrix.size() - 1)histogram[row][col] += histogram[row + 1][col];}}}int  rowMaxRectangle(vector<int>& heights) {// 利用升序栈int ret = 0;stack<int> _stack;for (int i = 0; i < heights.size();) {if (_stack.empty() || heights[_stack.top()] <= heights[i])_stack.push(i++);else {int val = heights[_stack.top()];_stack.pop();int leftPos = _stack.empty() ? 0 : (_stack.top() + 1);ret = max(ret, val * (i - leftPos));}}while (!_stack.empty()) {int val = heights[_stack.top()];_stack.pop();int leftPos = _stack.empty() ? 0 : (_stack.top() + 1);ret = max(ret, val * ((int)heights.size() - leftPos));}return ret;}int maxRectangle(vector<string> &matrix) { // 求最大长方形 if (matrix.empty())return 0;vector<vector<int>> histogram(matrix.size(), vector<int>(matrix[0].size(), 0));buildHistogram(matrix, histogram);int ret = 0;for (auto heights : histogram)ret = max(ret, rowMaxRectangle(heights));return ret; }