[LeetCode][JavaScript]N-Queens II

N-Queens II

Follow up for N-Queens problem.

Now, instead outputting board configurations, return the total number of distinct solutions.

https://leetcode.com/problems/n-queens-ii/

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有了上一道的基础，这道就是送的。

http://www.cnblogs.com/Liok3187/p/4558502.html

test case也不是很强，我本地试14就要等10多秒，再往上就炸了。

/**
 * @param {number} n
 * @return {string[][]}
 */
var totalNQueens = function(n) {
    var map = [];
    var count = 0;   
    buildMap();
    dfs(0);
    return count;

    function dfs(row){
        var i = 0, j = 0;
        var candidates = [];
        for(i = 0; i < n; i++){
            if(canPlaceQueen(row, i)){
                candidates.push(i);
            }
        }
        if(row === n -1 && candidates.length !== 0){ //found
            map[row][candidates[0]] = 'Q';
            count++;
            map[row][candidates[0]] = '.';
            return;
        }
        for(i = 0; i < candidates.length; i++){
            map[row][candidates[i]] = 'Q';
            dfs(row + 1);
            map[row][candidates[i]] = '.';
        }
    }
    function canPlaceQueen(x, y){
        var cell = "";
        var i = 0, j = 0;
        for(i = 0; i < n; i++){
            cell = map[i][y];
            if(cell === 'Q'){
                return false;
            } 
        }
        for(i = x, j = y; i >=0 && j >= 0; i--, j--){
            cell = map[i][j];
            if(cell === 'Q'){
                return false;
            } 
        }
        for(i = x, j = y; i >=0 && j < n; i--, j++){
            cell = map[i][j];
            if(cell === 'Q'){
                return false;
            } 
        }
        return true;
    }
    function buildMap(){
        var i = 0, j = 0;
        for(i = 0; i < n; i++){
            map[i] = [];
        }
        for(i = 0; i < n; i++){
            for(j = 0; j < n; j++){
                map[i][j] = '.';
            }
        }
    }
};