Leetcode 437. 路径总和 III (Day 16)JavaScript

var pathSum = function(root, targetSum) {
    let count=0,sum=0;
    const idx=new Map();
    function dfs(root){
        if(!root) return;
        idx.set(sum,(idx.get(sum)??0)+1);
        sum+=root.val;
        const j=sum-targetSum;
        count+=idx.get(j)??0;
        dfs(root.left);
        dfs(root.right);
        sum -= root.val;
        idx.set(sum, idx.get(sum) - 1);
        
    }
    dfs(root);
    return count;
};

算法核心同题560 前缀和+哈希表
注意:要回溯! 如果不回溯左子树会污染右子树,同时回溯时要与添加时对称

idx.set(sum,(idx.get(sum)??0)+1);
sum+=root.val;
//对称
sum -= root.val;
idx.set(sum, idx.get(sum) - 1);