542 01matrix

class Solution {
public:
    vector<vector<int>> updateMatrix(vector<vector<int>>& matrix) {
        int a=matrix.size();//行数
        int b=matrix[0].size();//列数
        vector<vector<int>>ans(a,vector<int>(b,INT_MAX-1));
        for(int times=0;times<=1;times++){//循环两次，第一次从左上向右下；第二次从右下向左上
            for(int i=times?a-1:0;times?i>=0:i<a;times?i--:i++){
                for(int j=times?b-1:0;times?j>=0:j<b;times?j--:j++){
                    if(matrix[i][j]==0){
                        ans[i][j]=0;
                    }else{
                        if(j-1>=0&&ans[i][j-1]!=INT_MAX-1&&ans[i][j]>ans[i][j-1]+1){//向左
                            ans[i][j]=ans[i][j-1]+1;
                        }
                        if(j+1<b&&ans[i][j+1]!=INT_MAX-1&&ans[i][j]>ans[i][j+1]){//向右
                            ans[i][j]=ans[i][j+1]+1;
                        }
                        if(i-1>=0&&ans[i-1][j]!=INT_MAX-1&&ans[i][j]>ans[i-1][j]+1){//向上
                            ans[i][j]=ans[i-1][j]+1;
                        }
                        if(i+1<a&&ans[i+1][j]!=INT_MAX-1&&ans[i][j]>ans[i+1][j]+1){//向下
                            ans[i][j]=ans[i+1][j]+1;
                        }
                    }
                }
            }
        }
        return ans;
    }
};